Causal association between B cell count and psoriasis using two‐sample Mendelian randomization

Abstract To investigate the causality between B cell count and psoriasis by Mendelian randomization (MR). Collected B cell count and psoriasis data from IEU Open GWAS Project. Employed inverse variance weighting (IVW), MR‐Egger, WM, weighted mode for analysis, ensuring result robustness. Assessed horizontal pleiotropy with MR‐Egger, detected outliers using MR‐PRESSO and examined instrumental variables heterogeneity with Cochran's Q‐test. The IVW method suggested an association between a genetically predicted memory B cell count and the risk of psoriasis vulgaris. IVW results also showed no causality between other exposure factors and the corresponding outcomes. Also, the global test of MR‐PRESSO analysis showed a significant association between a genetically predicted transitional absolute B cell count and the lower risk of psoriasis vulgaris. MR‐Egger regression showed that horizontal pleiotropy did not influence the analysis results. We found that memory B cell absolute counts are associated with a lower risk of psoriasis. These data further elucidate the role of memory B cells in psoriasis and provide new options for psoriasis treatment.

systematic longitudinal analysis of B-cell subpopulations and corresponding immunoglobulin (Ig) levels to investigate the potential impact of B cells in psoriasis, finding the alteration of B-cell subsets, particularly up-regulation of transitional B cells (also known as TrB cells), short-lived plasmablasts (PB), and IgA might be part of an inflammation-induced compensatory mechanism in psoriasis.Nevertheless, these studies contain reverse causality and confounding factors.

Mendelian randomization (MR) is a new method based on
Mendel's laws of inheritance and instrumental variable estimation methods that use genetic variants to assess causal relationships and how modifiable exposures influence different outcomes. 7A genetic variant can be considered as an instrumental variable for a given exposure if it satisfies the instrumental variable assumptions 1,8 : it does not affect the outcome 2 ; it is not associated with the outcome due to confounding pathways 3 ; it is associated with the exposure.3][14][15] However, the relationship between B cell count and psoriasis using the MR method has not yet been tested.
In this study, we applied an MR analysis to investigate whether B cell count can be used as a predictive factor for psoriasis, which should be further explored through MR analysis.
p < 5 × 10 −6 .Then, SNPs with a minimum minor allele frequency (MAF) >0.01 were screened, and SNPs with linkage disequilibrium R 2 < 0.001 or window size = 10,000 kb were excluded to avoid confounding effects.When the selected IVs are not present in the summary data of the outcome, SNPs with high LD (R 2 > 0.8) with the IV as proxy SNPs for replacement were screened.Next, the F-value for each SNP in the IV was calculated to assess IV strength, excluding potential weak instrument bias between the IV and exposure factors, using the following formula: , where R 2 represents the proportion of exposure variance explained by the SNP in the IV.The requirement for the F-value is >10.

| Mendelian randomization and sensitivity analysis
The MR analysis followed the Strengthening the Reporting of Observational Studies in Epidemiology using Mendelian Randomization (STROBE-MR) guidelines. 17The analysis process adhered to three assumptions of MR studies (Figure 1). 8Various methods were used to estimate the causal relationship between the exposure and outcome, including weighted median (WM), inverse variance weighting (IVW), simple model, and MR-Egger, weighted model.This analysis primarily employed the IVW method to evaluate the causal relationship between exposure and outcome risk by calculating the odds ratio (OR) and 95% confidence interval (CI). 18l analyses were performed using R 4.1.2software, incorporating packages such as TwoSampleMR and MR-PRESSO.Heterogeneity was tested using Cochran's Q, and pleiotropy was examined through MR-Egger regression of intercept values.Scatter plots and sensitivity analysis plots were used for visualization. 19As there are four outcome factors in this study, the false discovery rate (FDR) correction method was applied to correct for multiple testing, with PFDR <0.05 considered statistically significant.Scatter plots and sensitivity analysis plots were used for visualization.

| The causal effect of B cell count on psoriasis
The association between genetically predicted absolute B cell count, memory B cell count, switched memory B cell count, transitional absolute B cell count and un-switched memory B cell count and the risk of psoriasis, plaque psoriasis is shown in Table 1.IVW method suggested an association between a genetically predicted memory B cell count and the risk of psoriasis vulgaris [OR (95% CI): 0.88 (0.78-0.99), p = 0.04] (see Table 1, Figure 2).IVW results showed no causality between other exposure factors and the corresponding outcomes (see Table 1, Figures S1-S4).
Results from MR-Egger regression showed that horizontal pleiotropy did not influence the analysis results; no outliers were observed (Table 2), which suggests that this data is robust.

| DISCUSS ION
B cells are a key player in the adaptive immune response and are responsible for humoral immunity in mammals.These immune cells can promote inflammatory processes by producing cytokines that act on immune cells that mediate inflammation. 20Dysregulation of B-cell function can lead to severe consequences for the host, and different B-cell malignancies have been described so far 21,22 ; still, B cells have largely been neglected in studies of psoriasis, potentially owing to their limited presence in involved psoriatic skin.5][26] However, most of these studies were retrospective and observational studies that did not consider confounding factors such as possible genetic influence on disease development.This study investigated the causality between B cell count and psoriasis using MR, a method that applies to the use of genetic variation to address causal questions about how modifiable exposures influence different outcomes.We found that memory B cell count is associated with the lower risk of psoriasis.These data to the respective antigen, provide a rapid response to subsequent autoantigen exposure. 27Previous studies have confirmed that memory B cells are formed under inflammatory conditions, such as during flare-ups of autoimmune diseases (ADs).These memory B cells can migrate into inflammatory tissues, where they are relatively protected from therapeutic interventions. 28For example, in patients with multiple sclerosis (MS), memory B cells are a major source of lymphotoxin and TNFα. 29Additionally, a trial of atacicept in patients with rheumatoid arthritis (RA) observed a transient increase in memory B cells. 30In systemic lupus TA B L E 1 Association between genetically predicted B cell count (absolute count of B cells, memory B cell count, switched memory B cell count, transitional absolute B cell count and un-switched memory B cell count) and the risk of psoriasis, plaque psoriasis.moderate-to-severe psoriasis found that memory B cells and total PC were significantly lower in patients with psoriasis.These reports highlight the significant role of memory B cells in the pathogenesis of ADs and suggest their potential for treatment.
In 2000, Ochsenbein et al. 27 identified differentially expressed genes with altered expression in psoriasis lesions (n = 216 patients) and found several genes associated with B cells.In this study, we selected 11 SNPs to analyse the association between memory B cell count and psoriasis vulgaris.This is the first study that suggests a genetic effect of memory B cell count on the risk of psoriasis vulgaris.Considering that stimulated memory B cells contribute to inflammation by cytokine expression and antigen presentation to T cells in several cutaneous ADs, 32 we suspect this type of cells may be involved in psoriasis.Previous studies utilizing MR have shown that immune cells, particularly memory B cells, play a significant role in the risk of systemic ADs, such as Sjögren's syndrome (SS). 33This finding implies that psoriasis and other ADs may share a common signalling pathway associated with memory B cell counts.However, further clinical data are necessary to confirm these results.
This study found no association between other B cell subsets and psoriasis.However, some observational studies reported different findings that might be taken into consideration.For example, Yanaba et al. 24 identified the contribution of B cells to the pathogenesis of psoriasis and found direct evidence that regulatory B cell subset cells, a type of B cell subset that suppress immunopathology by prohibiting the expansion of pathogenic T cells and other pro-inflammatory lymphocytes, regulate imiquimod-induced skin inflammation and offer insights into regulatory B cell-based therapies for the treatment of psoriasis.In addition, transitional B cells are assumed to play a role in ADs, such as SLE and psoriasis. 25More recently, Kahlert and his team 6 found that alteration of B-cell subsets, particularly up-regulation of transitional B cells, might be part of an inflammation-induced compensatory mechanism in psoriasis.A significant elevation of these cells seems to be controlled by NFATc1, a transcription factor that controls the activity of IL-10 production in psoriasislike skin inflammation. 34Furthermore, the multi-stage differentiation of B cells and the specific markers for IL-10-producing B cells remain inadequately defined. 35 After ensuring the robustness, heterogeneity, and generalizability of SNPs while avoiding their potential confounding effects and pleiotropy, 17, 11, 16, 21 and 13 IVs related to the absolute B cell count, memory B cell count, switched memory B cell count, transitional absolute B cell count, and un-switched memory B cell count were selected.After calculation, the mean F-statistic value of the IVs was 25.22, 24.55, 23.40, 23.75 and 24.91, with a minimum value of 20.99, 21.61, 21.04, 20.90 and 21.24, and a maximum value of 40.85, 42.24, 29.05, 33.13 and 33.29 for an absolute count of B cells, memory B cell count, switched memory B cell count, transitional absolute B cell count and un-switched memory B cell count, respectively.
shed new light on the role of B cells in psoriasis and open new ways for psoriasis treatment.Mature B cells recirculate through the blood and accumulate in the follicles of secondary lymphoid tissues.After stimulation with their cognate (self-) antigen, mature B cells eventually form shortlived PB, memory B cells or long-lived plasma cells (PC).Memory B cells, generated during T-dependent and T-independent responses

TA B L E 1
(Continued) F I G U R E 2 A causal association between memory B cell absolute count and the risk of psoriasis vulgaris.Forest plot image (A); leaveone-out plot (B); funnel plot image (C); scatter plot image (D).(a) Memory B cell absolute count versus 'non-cancer illness code self-reported: Psoriasis'; (b) memory B cell absolute count versus 'Psoriasis (vulgaris), strict definition'.
Additional research is necessary to elucidate the role of B cells in the pathogenesis of psoriasis.This study represents the first evaluation of the association between B cell count and psoriasis.Our study's strengths include the use of a rigorous MR design, multiple sensitivity analyses and largescale GWAS data.Limitations include the potential for unmeasured pleiotropy, residual confounding and the reliance on predominantly European ancestry populations, limiting generalizability.Future research could benefit from more diverse ethnic samples, functional validation of implicated genetic variants and exploration of potential mediating pathways linking B cell count to psoriasis.In conclusion, the present MR study did demonstrate that transitional absolute B cell count and memory B cell count are associated with psoriasis.Revealing the potential association of B cell count with the occurrence of psoriasis, which can provide clues for further studies of psoriasis diagnosis, prevention and treatment strategies.Also, further enrolling a bigger population to study the function of B cell count in the pathogenesis of psoriasis is warrant.AUTH O R CO NTR I B UTI O N S Zongfeng Zhao: Formal analysis (lead); writing -original draft (equal); writing -review and editing (equal).Jie Cheng: Resources (lead); writing -original draft (equal); writing -review and editing (equal).Jian Zhu: Data curation (equal); investigation (equal); methodology TA B L E 2 Results of IV heterogeneity test and multiplicity test.